Embodiments of a fixing apparatus to be installed in a copier or a printer include an endless belt, a ceramic heater that is in contact with an inner surface of the endless belt, and a pressure roller forming a fixing nip portion together with the ceramic heater through the endless belt. Upon performing continuous printing on a small-size sheet with an image forming apparatus including the fixing apparatus, the phenomenon of a gradual temperature increase in portions in the longitudinal direction of the fixing nip portion in which the sheet is not fed (non-sheet feeding portion temperature increase) occurs. If the non-sheet feeding portion has an excessively high temperature, parts in the apparatus may be damaged, and if printing is performed on a large-size sheet in a state in which a non-sheet feeding portion temperature increase has occurred, hot offset of toner may occur in areas corresponding to non-sheet feeding portions for a small-size sheet.
As an method for suppressing this non-sheet feeding portion temperature increase, forming heat generation resistive members on a ceramic substrate of a material having a positive temperature coefficient of resistance and arranging two conductive members on opposite ends in the lateral direction of the substrate so that current flows in the heat generation resistive members in the lateral direction of the heater (recording sheet conveyance direction) has been considered. This is based on the following idea: when a temperature increase occurs in the non-sheet feeding portions, the resistivity values of the heat generation resistive members in the non-sheet feeding portions increase, suppressing the current flowing in the heat generation resistive members in the non-sheet feeding portions, thereby suppressing heat generation in the non-sheet feeding portions. A positive temperature coefficient of resistance, which is a characteristic in which as the temperature increases, the resistance increases, is referred to as “PTC” (positive temperature coefficient) hereinafter.
However, a PTC material has a very low volume resistance, and thus, it is difficult to set the total resistance of the heat generation resistive members in one heater within a range that can be used with a commercial power supply. Therefore, PTC heat generation resistive members formed on a ceramic substrate are segmented into a plurality of heat generation blocks in the longitudinal direction of the heater, and in each heat generation block, two conductive members are arranged at opposite ends in the lateral direction of the substrate so that current flows in the lateral direction of the heater (recording sheet conveyance direction). Furthermore, Japanese Patent Application Laid-Open No. 2005-209493 discloses a configuration in which a plurality of heat generation blocks are electrically connected in series. This literature also discloses connecting a plurality of heat generation resistive members electrically in parallel between two conductive members to configure a heat generation block.